This invention relates to catalysts having improved activity for the production of hydrocarbons from hydrogen and carbon monoxide and to an improved hydrocarbon synthesis process. Specifically, this invention relates to a catalyst comprising cobalt supported on an inorganic oxide promoted by molybdenum and or molybdenum and zirconium.
The conversion of mixtures of carbon monoxide and hydrogen, for example synthesis gas or syngas, is commonly referred to as Fischer-Tropsch synthesis. Fischer-Tropsch synthesis was used extensively in Germany during World War II. There is considerable incentive for use of the process in the conversion of coal to liquid fuels and for conversion of natural gas to liquid fuels. Liquid fuels are more easily transported and utilized than coal. Conversion of natural gas to liquid makes transportation and storage more feasible. Sasol operates commercial Fischer-Tropsch plants in South Africa which employ an iron catalyst (see for example Oil and Gas Journal, Jan. 20, 1992, p. 53). A large commercial plant using Shell Oil technology has been recently placed in production in Malaysia. These commercial operations typically employ fixed-bed reactor systems.
Slurry phase reactors, especially slurry bubble column reactor systems (SBCR), for Fischer-Tropsch processes have received considerable attention in recent years. The slurry process has a number of advantages, including the ability of the reactor to handle the large heats of reaction and thereby control reaction temperature; the ability to convert low H.sub.2 /CO ratio synthesis gas without the need for a separate water-gas shift process step; and relatively low capital and operating costs. (See Hydrocarbon Processing, "Catalysts for Fischer-Tropsch," February 1990 pp. 59-68.) Slurry reactor systems are characterized by suspending the Fisher-Tropsch catalyst in an upflow of synthesis gas in a liquid medium. Basically, the process includes a finely divided catalyst suspended in oil that is mixed in a reactor (e.g. a SBCR) in the presence of synthesis gas. Early patents describing the slurry process are U.S. Pat. Nos. 2,438,029; 2,680,126; 2,852,350 and others. Slurry reactor systems are discussed in the article "Fisher-Tropsch Synthesis in the Slurry Phase," M. D. Schlesinger et al., Industrial Engineering Chemistry, Vol. 6, p. 1474 (1951). U.S. Pat. No. 4,252,736 discloses a process in which syngas is continuously bubbled through a column of Fisher-Tropsch catalyst suspended in oil.
In principle, all catalysts that are active for Fisher-Tropsch synthesis can be used in slurry reactor systems. The objective of catalyst choice is to obtain the highest possible selectivity of desired liquid hydrocarbon products and the highest possible activity. Iron catalysts have been preferred because of low cost and good activity. However, better catalyst-reactor systems are desired. U.S. Pat. No. 5,162,284 to Soled et al. describes a copper promoted cobalt manganese spinel catalyst.
Common Fischer-Tropsch catalysts are cobalt, and iron (see for example, "The Fischer-Tropsch Synthesis," by R. B. Anderson, Academic Press (1984), p. 2). Other Group VIII metals such as ruthenium and osmium are also active. Other metals that have been investigated as primary catalyst components include rhenium, molybdenum, and chromium, but these have very low or no activity and produce primarily methane.
The activity of supported cobalt catalysts can be enhanced, or the performance modified, by the addition of a variety of metals. Exemplary metals include copper (U.S. Pat. Nos. 5,302,622 and 5,162,284), cerium (U.S. Pat. Nos. 3,888,792; 4,657,885; 4,801,573 and 4,880,763), rhenium (U.S. Pat. Nos. 4,088,671; 4,558,030; 4,568,663; 4,801,573 and 4,880,763) and manganese (U.S. Pat. No. 5,162,284). Precious metals include platinum, iridium, ruthenium and rhodium (U.S. Pat. Nos. 5,302,622; 5,059,574 and 5,102,851). In addition to enhancing catalyst activity, promoters are added to achieve specific results, e.g., to enhance liquid hydrocarbon production, to suppress methane production, etc. See, for example, the discussion in U.S. Pat. No. 4,880,763. U.S. Pat. No. 5,302,622 to Chaumette, et al. references French Patent Application No. 91/07,634 that describes a catalyst containing cobalt, at least one additional element chosen from molybdenum and tungsten and at least one element chosen from elements including ruthenium and copper.
A series of Shell patents (U.S. Pat. Nos. 4,522,939; 4,499,209; 4,587,008 and 4,686,238) disclose supported cobalt-silica catalysts promoted with zirconium, titanium or chromium. These catalysts are designed for fixed bed operation. Their effectiveness is dependent on the specific nature of metal incorporation on the support, i.e., by sequential impregnations and/or kneading.
U.S. Pat. Nos. 4,801,573 and 4,880,763 recite the use of small amounts of promoter oxides chosen from elements in Groups IIIB, IVB and VB (including zirconia but no promotional effect on either activity or selectivity was shown).
In view of the known tendency of molybdenum containing catalysts to lower Fisher-Tropsch synthesis activity and to increase methane production or the use of molybdenum as a promoter only with additional promoters, it was surprising to discover that cobalt catalysts could be effectively promoted to excellent activity and improved liquid hydrocarbon selectivity by incorporation of molybdenum and/or molybdenum together with zirconium. This invention shows such an improved catalyst in a slurry phase reactor and catalyst.